1. Field of the Invention
The present invention relates to a method of manufacturing a plated steel sheet with a Zn--Cr alloy plating which is excellent in adhesion properties.
2. Description of the Related Art
Zinc plating has sacrificing corrosion protecting property for steel sheets. The sacrificing corrosion protecting property is exhibited under environments in a variety of applications. Zinc-plated steel sheets have been used in various fields of automobiles, home appliances, and construction materials as major application fields of steel sheets.
In recent years, however, strong demand has arisen for further improving the corrosion resistance property of zinc-plated steel sheets, and particularly, electrical zinc-plated steel sheets mainly in the field of automobiles. To meet this demand, a steel sheet with a zinc alloy plating of, e.g., Zn--Ni or Zn--Fe has been developed, and its production quantity has been increasing. This steel sheet has become a major one of all steel products.
Attempts have been made to improve the properties of steel sheets with zinc alloy platings. In particular, it is reported that the corrosion resistance property of steel sheet with Zn--Cr alloy plating is excellent (Jpn. Pat. Appln. KOKAI Publication Nos. 63-243295 and 1-162794). There are also proposed methods of manufacturing steel sheet with Zn--Cr alloy plating (Jpn. Pat. Appln. KOKAI Publication Nos. 64-79393, 3-120393, and 64-55398).
Electrical zinc-plated steel sheets (including plated steel sheets with its various alloys) are generally manufactured using an acidic plating bath such as a sulfuric acid bath or a hydrochloric acid bath. In the manufacture of a plated steel sheet with a Zn--Cr alloy plating, this plated steel sheet cannot be obtained if chromium ions are simply added to such an acidic plating bath.
According to Jpn. Pat. Appln. KOKAI Publication No. 1-79393, it is described that Zn--Cr plating can be performed in a plating bath containing only zinc and chromium ions (partially including inorganic salts) at a high current density of 150 A/dm.sup.2 or more. Inspite of this, at present, electrical zinc platings (including zinc alloy platings) are generally manufactured at a current density of 50 to 150 A/dm.sup.2, and it is difficult to apply a high current density of 150 A/dm.sup.2 or more in practice. Moreover, in plating at a high current density, diffusion of metal ions subjected to plating cannot generally follow up the plating rate. The hydrogen ions are subjected to a reducing reaction instead, and the pH of a portion near the plated surface is increased by this reducing reaction to produce and precipitate a hydroxide of the metal ions, thereby forming a black plating film having poor adhesion properties. This phenomenon produces so-called burnt deposits. This state degrades the plating adhesion property, and a practical plating film cannot be obtained. In fact, according to an examination of the present inventors, when plating was performed in a plating bath containing only zinc and chromium ions at a current density of 150 A/dm.sup.2 or more, a plating burnt deposit was formed to fail to obtain a practical plating film although chromium was contained in the plating film.
According to Jpn. Pat. Appln. KOKAI Publication No. 3-120393, it is decribed that plating using a Zn--Cr alloy can be performed by adding a tartrate in a sulfuric acid bath containing zinc and chromium ions as major components. According to an examination of the present inventors, however, even if a tartrate was added to the sulfuric acid bath, chromium hardly coprecipitated, and a Zn--Cr plating film was not obtained.
According to Jpn. Pat. Appln. KOKAI Publication No. 64-55398, it is described that plating using a Zn--Cr alloy can be performed by adding a polyoxyalkylene derivative. According to an examination of the present inventors, polyethylene glycol as a kind of polyoxyalkylene derivative was added to obtain a Zn--Cr plating film. However, the plating adhesion properties were degraded with an increase in Cr content. In particular, when the Cr content was 15% or more, the plating adhesion properties were greatly degraded to fail to obtain a practical plating film.
As described above, according to the conventional plating methods using Zn--Cr alloy plating, it is difficult to obtain Zn--Cr plating films having excellent adhesion properties at a high Cr content.